Optical support structures in imaging systems must possess excellent stability during testing and operations. Two of the most important structural parameters affecting stability are stiffness and Coefficient of Thermal Expansion (CTE). For composite materials, Coefficient of Moisture Expansion (CME) is also important.
The stiffness of a structural member is related to the modulus of elasticity, or modulus, for the structural material. The weight of the structural material is related to the material density. The specific modulus of a structural material is the modulus divided by the density. The specific modulus for a structural material is proportional to the stiffness to weight ratio. The specific modulus is more convenient to compute and is therefore used as a comparison parameter for structural materials.
Carbon fiber reinforced plastic (CFRP), often referred to as composite laminate materials, is a well known class of materials that is used in structural applications ranging from aircraft to fishing poles due to its high stiffness to weight ratio and high strength to weight ratio, which can be much higher than metals. The material consists of various layers, or plies, oriented and stacked in a prescribed pattern (analogous to plywood) tailored to meet the structural requirements of the member. Each layer of the laminated sheet consists of reinforcing fibers such as carbon, glass, or boron embedded in a plastic resin.
It has been demonstrated that it is possible to achieve laminates possessing near-zero CTE in one laminate direction with high stiffness and low weight using carbon fibers. The near-zero CTE attainable is equal to or lower (better) than that of Invar, a steel alloy with the lowest CTE possessed by a traditional metal material (0.54 .mu.m/m/.degree.C.).
It has also been demonstrated that it is possible to achieve laminates possessing near-zero CME in one laminate direction with high stiffness and low weight. Metallic materials do not absorb atmospheric moisture (are not hygroscopic), therefore, the CME of any metal, including Invar, is 0 .mu.m/m/%.DELTA.M. Currently available instruments for measuring CME are capable of accuracies of .+-.20 .mu.m/m/%.DELTA.M. For this reason, near zero CME is defined herein as within .+-.20 .mu.m/m/%.DELTA.M.
The stiffness to weight ratio of Invar is well below that which can be achieved with near-zero CTE and near-zero CME carbon fiber composite laminates.
The CTE of a laminate, as well as its strength and stiffness, is determined by the type of fiber selected, the orientation of each layer of those fibers, and the fiber volume fraction (ratio of the volume of fibers to the total volume of the laminate). Carbon fibers have a negative CTE in the direction of the fiber. In general, the CTE varies with the modulus of the fiber, i.e. the stiffer the fiber, the more negative the CTE. Resins have a positive CTE. For a given fiber, resin, and fiber volume fraction, it is necessary to orient some or all of the carbon fibers at some .+-.Theta angle relative to the reference direction in order to produce a laminate with a near-zero CTE in that direction. However, as the Theta angle is increased, the modulus of the laminate decreases. Currently carbon fibers in the 210-345 GPa (10.sup.9 Pascal) modulus range produce the highest stiffness to weight ratio laminates with near-zero CTE. Higher modulus carbon fibers (520-900 GPa) can also be used to achieve near-zero CTE laminates. Again, these fibers generally have a larger negative CTE than the 210-345 GPa modulus fibers. Consequently, larger ply angles (.+-.Theta) are required to achieve near-zero CTE. The resulting laminate modulus and therefore, specific modulus, is equal to or less than the 210-345 GPa modulus carbon fibers. The high negative CTE of the high modulus fibers prohibits increasing the laminate specific modulus above that attained by the 210-345 GPa fibers.
The CME of the laminate is also determined by the type of fiber selected, the orientation of each layer of fibers, the fiber volume fraction, and the CME of the resin. The resins used for composite laminates absorb atmospheric moisture. Carbon and boron fibers, in general, do not absorb atmospheric moisture. As the resin absorbs or desorbs moisture, the laminate will expand or contract in length. The amount of expansion or contraction per unit length and per unit change in moisture content is defined as the Coefficient of Moisture Expansion (CME). For a given fiber, resin, and fiber volume fraction, it is necessary to orient some or all of the carbon fibers at some .+-.Theta angle relative to the reference direction in order to produce a laminate with a near-zero CME in that direction. Currently carbon fibers in the 210-900 GPa (10.sup.9 Pascal) modulus range produce high stiffness to weight ratio laminates with near-zero CME.
The current state of the art carbon laminates can be utilized to produce structural components with the CTE equivalent to or lower than Invar, at a much higher specific modulus, or structural components with CME equal to the zero CME of Invar at a much higher specific modulus. Currently, a carbon-fiber laminate does not exist that possesses both near-zero CTE and near-zero CME. There exists a need for structural components with near-zero CTE and simultaneously, near-zero CME while maintaining higher specific modulus. This need to achieve near-zero CTE simultaneously with near-zero CME is driven by a demand to further reduce hygrothermal excursions by structural components operating in an environment with changing temperature and relative humidity.